Artificial intervertebral disc

ABSTRACT

An artificial intervertebral disc includes housing members including spaced inner surfaces facing each other and oppositely facing outer surfaces for engaging spaced apart vertebral surfaces. Bearing surfaces extend from each of the inner surfaces for engaging each other while allowing for low friction and compression resistant movement of the housing members relative to each other while under compression. Load sharing pads are disposed between the inner surfaces and about at least a portion of the bearing surfaces for sharing absorption compressive loads with the bearing surfaces while limiting the relative movement of the housing members.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates generally to a spinal implant assembly forimplantation into the intervertebral space between adjacent vertebralbones to provide stabilization and continued postoperative flexibilityand proper anatomical motion. More specifically, the present inventionrelates to an artificial intervertebral disc, sometimes referred to asan intervertebral spacer device, for functioning as a load sharing andbearing device for replacement of the damaged, decayed, or otherwisenonfunctioning intervertebral disc.

2. Background of the Invention

The spine is a complex structure consisting of multiple flexible levels.Each level consists of a system of joints defined by adjacent vertebralbones. The system of joints includes intervertebral discs, which are atwo-part structure. The disc consists of a nucleus and an annulus. Thesystem allows motion while the facet joints add posterior stabilizationto the spinal column. The disc allows motion and cushioning to thejoint.

The complex system of the joint is subjected to varying loads andproblems over time, including disc degeneration due to a variety ofreasons. Disc degeneration can be attributed to aging, damage due toexcessive loading, trauma, and other anatomical issues. Facet joints ofthe structure can be compromised due to the same reasons, as well as dueto arthritic changes. Severe joint degeneration and failure can oftencause sufficient pain to require surgical intervention.

The current standard method of treatment for severe pain caused by spinejoint problems is fusion at the damaged level of the spine. Thetreatment, when successful, fuses the damaged section into a single massof bone. The fusion of the joint eliminates motion of the joint, therebyreducing or eliminating pain at that level. Success rates for painelimination are very high for this method of treatment. However, sincethe entire spine works as a system, fusion results in complications.

Elimination of motion at the spine alters the biomechanics of the spineat every other level. If one level is fused, then loads are absorbed byone less disc into a system not designed for such change. Thus, theremaining discs must redistribute loads, each disc absorbing a greaterload. In addition, the spine flexes to absorb loads. A fusion alters themeans by which the spine flexes, which also increases the loads on theremaining healthy discs. In turn, it is well understood that acomplication of fusion is that additional fusions may be required in thefuture as the other discs deteriorate due to the altered biomechanics ofthe spine. In other words, short-term pain relief is exchanged forlong-term alterations of the spine, which, in turn, usually requirefurther surgery.

There are numerous prior art patents addressing the issue of discreplacement. The U.S. Pat. Nos. 6,443,987 B1 and 6,001,130, both toBryan, disclose polymer composite structures for cushioningintervertebral loads. The U.S. Pat. Nos. 5,258,031 to Salib, et al. and5,314,477 to Marnay disclose ball and socket type implants addressingthe issue of intervertebral mobility. These patents are exemplary of afirst approach using an elastomer as a motion and dampening structureand a second approach utilizing a ball and socket joint to create amoving pivot joint. There are many variations on these concepts, whichinclude mechanical springs and more complex structural mechanisms. Asignificant portion of the prior art addresses the issues ofintervertebral motion but do not address anatomical loadingconsiderations.

The current state of prior art artificial intervertebral discs areassociated with various problems. For example, a number of implantsconstructed from polymers are of insufficient strength to workeffectively in the higher loading areas, such as the lumbar spine. Suchpolymers often take compressive sets so that the original height of theimplant decreases over time. A surgeon must either compensate for thecompression by initially using a larger polymer prosthesis and estimatecompression or use the appropriately sized polymer prosthesis and latersurgically replace the same once the irreversible compression of theprosthesis is unacceptable.

Implants constructed with ball and socket joints severely restrict oreliminate shock cushioning effect of a normal disc. This implant canprovide motion, but biomechanically, the ball and socket jointnegatively affects other healthy discs of the spine. The result can belong-term problems at other levels of the spine, as seen with thecurrent treatment of fusion.

Other implants, not discussed above, utilize bearing surfaces usuallyhaving polyethylene bearing against metal interfaces. Polyethylene as abearing surface is problematic in large joint replacement due to thewear properties of the material. Since artificial discs are intended tobe implanted over long periods of time, such wear can be highly damagingto surrounding tissue and bone.

In view of the above, it is desirable to provide a solution tointervertebral disc replacement that restores motion to the damagednatural disc area while allowing for motion as well as cushioning anddampening, similar to the naturally occurring disc. In addition, it ispreferable to allow such motion, cushioning, and dampening whilepreventing a polymer or elastomeric material from experiencing therelatively high compressive loads seen in the spine. It is alsopreferable to allow a bearing surface to share the spinal loads with thepolymer and elastomeric material. Finally, it is preferable to controlchanges to the artificial motion intraoperatively to adjust foranatomical conditions.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided anartificial intervertebral disc including housing members having spacedinner surfaces facing each other and oppositely facing outer surfacesfor engaging spaced apart vertebral surfaces. Bearing surfaces extendfrom each of the inner surfaces for engaging each other while allowingfor low friction and compression resistant movement of the housingmembers relative to each other while under compression. Load sharingpads disposed between the inner surfaces and about at least a portion ofthe bearing surfaces share absorption of compressive loads with thebearing surfaces while controllably limiting the relative movement ofthe housing members.

The present invention further provides a method of assembling anartificial intervertebral disc in vivo by inserting upper and lowerhousing members into an intervertebral space and disposing cushioningpads between the inner surfaces of the housing members, placing the padsin compression. A pair of disc members are inserted between the innersurfaces of the plates, the disc members having abutting low frictionsurfaces therebetween. The disc members effectively are surrounded bythe pads whereby the disc members and pads are under compressive forces.

Additionally, a method of separating opposing vertebrae at anintervertebral space includes the steps of engaging artificial bearingsurfaces between the intervertebral spaces while allowing low frictionand compression resistant movement of the bearing surfaces relative toeach other, sharing absorption of the compressive forces with at leastone load bearing pad disposed about at least a portion of the bearingsurfaces, and limiting the relative movement of the bearing surfaces.

DESCRIPTION OF DRAWINGS

Other advantages of the present invention can be readily appreciated asthe same becomes better understood by reference to the followingdetailed description when considered in connection with the accompanyingdrawings wherein:

FIG. 1 is a side perspective view of a preferred embodiment of thepresent invention;

FIG. 2 is a side exploded view of the embodiment shown inn FIG. 1;

FIG. 3 is a side perspective view of a second embodiment of the presentinvention;

FIG. 4 is a perspective view of a lower disc constructed in accordancewith the present invention;

FIG. 5 is a side view of an upper disc constructed in accordance withthe present invention;

FIG. 6 is a top perspective view of an upper housing member made inaccordance with the present invention;

FIG. 7 is a top plan view of a lower housing member made in accordancewith the present invention;

FIG. 8 is a side perspective view of a third embodiment of the presentinvention;

FIG. 9 is a perspective view of the present invention with the tophousing member removed;

FIG. 10 is a perspective view of an alternative pad configuration of thepresent invention;

FIG. 11 is a perspective view of a further alternative embodiment of thepad member;

FIG. 12 is a further alternative embodiment of the present invention;

FIG. 13 is an exploded side perspective view of the embodiment shown inFIG. 12;

FIG. 14 shows an alternative embodiment of the housing members of thepresent invention;

FIG. 15 shows a further alternative embodiment of the housing members ofthe present invention;

FIG. 16 is an exploded view of a further embodiment of the presentinvention demonstrating a bayonet type locking of a disc member to ahousing member;

FIG. 17 is a perspective view of the disc member utilizing the bayonetlocking mechanism to lock the disc member within a housing member;

FIG. 18 is an exploded view of a disc member and housing member showinga further embodiment of a locking mechanism for locking the disc memberwithin the housing member;

FIG. 19 is a perspective view showing the disc member locked within thehousing member;

FIG. 20 is a perspective view of the a further embodiment of the housingmember;

FIG. 21 is a cross sectional view taken along line 21-21 in FIG. 20;

FIG. 22 is a perspective view of a load sharing pad member includingflanges for locking engagement in the recesses of the housing membershown in FIGS. 20 and 21; and

FIG. 23 shows a further embodiment of a locking mechanism made inaccordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

An artificial intervertebral disc constructed in accordance with thepresent invention is generally shown at 10 in the figures. Likestructures of various embodiments are indicated by primed numerals inthe Figures. The invention is an artificial intervertebral disc,sometimes referred to by other terminology in the prior art such asintervertebral spacer device, or spinal disc for replacement of adamaged disc in the spine. The invention restores motion to the damagednatural disc that allows for motion as well as cushioning and dampening.As described below in more detail, the present invention also allowschanges to the artificial disc motion intraoperatively to adjust forspecific anatomical conditions.

Referring to the Figures, the disc 10 includes an upper housing membergenerally shown at 12 and a lower housing member generally shown at 14.The housing members 12, 14 include spaced inner surfaces 16 and 18facing each other and oppositely facing outer surfaces 20, 22 forengaging spaced apart vertebral surfaces. A pair of bearing surfaces 24,26 extend from each of the inner surfaces 16, 18 for engaging each otherwhile allowing for low friction and compression resistant movement ofthe housing members 12, 14 relative to each other while undercompression. As shown in the various Figures, the bearing surfaces areintegral with disc members 28, 30. Alternatively, the bearing surfaces24, 26 can be surfaces on projections that are integral with and extendfrom the housing members 12, 14, per se. The housing members 12, 14 canbe made from various materials including metals, such as titanium, aswell as ceramics, and plastics. If integral with the bearing surfaces24, 26, the housing members 12, 14 can be made from the preferredmaterial for the bearing discs 28, 30 as discussed above. Based on thisteaching, various other configurations can be made by those skilled inthe art incorporating the present invention.

The upper and lower bearing surfaces 24, 26 engage each other whendisposed correctly opposite each other. The configuration creates athree-dimensional bearing surface. As discussed below, the bearingsurfaces 24, 26 are disposed on noncompressible discs or the like,thereby providing structure for absorbing compressive loads placed onthe outer surfaces 20, 22 of the housing members 12, 14.

Load sharing pads generally shown at 31 and specifically indicated aspads 32 and 34 in FIGS. 1 and 2 are disposed between the inner surfaces16, 18 and about at least a portion of the bearing surfaces 24, 26 forsharing absorption of compressive loads with the bearing surfaces 24, 26while limiting relative movement of the housing members 12, 14. Morespecifically, under in vivo loading conditions, the centralized bearingsurfaces 24, 26 not only provide for three-dimensional movementrelatively between the housing members 12, 14, but also share with theload sharing pads 32, 34 the function of distributing compressive loadson the device 10 to provide a system for motion and effective loaddistribution. The centralized low friction and compression resistantbearing surfaces 24, 26 allow full motion in multiple planes of thespine while the load distributing damper and cushioning pads 32, 34simultaneously share the load. Critical is the function of the pads 32,34 sharing the load with the bearing surfaces 24, 26. Although the pads32, 34 can be compressible, the compression is limited by thenoncompressibility of the bearing surfaces 24, 26. Likewise, althoughthe bearing surfaces allow for motion in multiple planes, the pads 32,34 are fixedly secured to the housing members 12, 14, thereby allowingfor a degree of flexibility and therefore movement of the housingmembers 12, 14 relative to each other, yet limiting such movement. Intotal, each element, the bearing surfaces 24, 26, and pads 32, 34, allowfor movement, yet limit such movement, whether it is the slidingmovement of the bearing surfaces 24, 26 or the cushioning movementallowed by the pads 32, 34. Each element allows for relative movement,yet each element limits the movement of the other element of the system.

In view of the above, the system allows restoration of normal motionwhile maintaining load cushioning capabilities of a healthy disc. Thisis particularly apparent with motion of the spine. Any rotation of theupper and lower housing members 12, 14 causes the load distributingdampening and cushioning pads 32, 34 to absorb some of the load.

As shown in the various figures, the bearing surfaces 24, 26 can includea concave surface portion on one of the upper or lower disc members 28,30, and a convex surface portion on the other. The concave surface isseated within the convex surface for sliding movement relative theretoeffectively resulting in relative pivoting motion of the housing members12, 14, which compresses at least a portion of the load sharing pads 32,34 while extending at least a portion of the oppositely disposed loadbearing pad 32, 34. Alternatively, either one of the top and bottom discmembers 28, 30 can have either of the convex or concave surfaces.

The disc members 28, 30 can be made from a composition that isnoncompressible. Such compositions can be selected from the groupincluding ceramics, plastics, and metal bearing materials, such ascobalt and chrome. Alternatively, the housing members 12, 14 can includeprojections wherein the disc members 28, 30 are effectively integralwith the housing members 12, 14. In this situation, the entire housing,including the projections having the bearing surfaces 24, 26 thereon,can be made from the noncompressible material preferably a ceramic. Asstated above, alternative configurations can be made by those skilled inthe art once understanding the present invention.

The load sharing pads 32, 34 can be in various configurations shown inthe Figures, such as paired pads 32, 34 shown in FIGS. 1-3.Alternatively, the device 10 can include four oppositely disposed pads38, 40, 42, 44 as shown in FIG. 10. A further embodiment of theinvention is shown in FIG. 11, wherein a single pad 46 substantiallycovers the surface 18′″″of the housing member 14′″″. The pads cancontour to the shape of the housing members such as shown in FIGS. 12,13, wherein the pad member 48 is an annular pad member disposed with aannular housing 12″″″, 14″″″. The selection of such housing members 12,14 and pad members 31 can be determined based on the location of theplacement of the device 10 as well as the spacing conditions between thevertebrae and load bearing necessities depending on the level of thespine being addressed. In other words, different shaped devices, such asthe round shaped housing members shown in FIG. 12 can be used forplacement between smaller discs, such as cervical spines whereas morerectangular shapes, such as the housing members shown in FIGS. 1-11 canbe used in between lumbar vertebrae.

The load sharing pads 31, in which ever shape they are configured, areelastic for allowing relative twisting movement between the housingmembers 12, 14 effecting relative three-dimensional movement between thehousing members 12, 14, while limiting the movement and preventingcontact between the housing members 12, 14 except for the contactbetween the bearing surfaces 24, 26. By elastic, it is meant that thepad members 31 are compressible and stretchable, yet provide a selfcentering effect on the assembly with specific regard to the housingmembers 12, 14, as well as the bearing surfaces 24, 26. Deflection orrotation of the forces created due to relative movement of the bearingsurfaces 24, 26, and likewise the housing members 12, 14, forces thepads 31 to act in such a way to counter the force, thus allowing aunique self-centering capability to the assembly 10. While in an idealsituation, wherein the patient's facets are uncompromised and ligamentalbalances are intact, this self-centering aspect may not be completelynecessary. In other words, the patient's anatomy may still providestabilization and specifically, ligaments may provide self-centering.However, ligamental imbalance, and damaged facets would normally make anartificial disc questionable, at best, with use of the currenttechnology that is available. In such cases, having the ability toself-center and restrict motion (the pads 31 of the present inventionare elastic and thus restrict motion by stretching and returning torest), the possibility of extending indications to patients currentlyconsidered outside of the scope of artificial disc technology will behighly advantageous.

The pads 31 of the present invention provide further advantages to theinvention. A key advantage is the ability to adjust the pads 31 topatient and surgeon requirements. In such cases wherein range of motionneeds to be restricted due to compromised facets, a harder, less elasticpad can be inserted between the housing members 12, 14. Since this lesselastic pad would move and stretch less, the disc would be automaticallyrestricted in motion. This method of adjusting pads can be doneintraoperatively to compensate for surgical and patient conditions. Toone skilled in the art, one can fine-tune the assembly 10 to a patient'sand surgeon's needs with multiple pads of different properties ormaterials.

The pads 31 are made from a polymer or elastomer that allows deflectionunder load. Examples of such polymers and elastomers are silicone,polyurethane, and urethane composites. As discussed above with regard toflexibility or elasticity, the content and composition of the pads 31are adjustable. A highly dense material creates a very rigid disc, whilea very soft material creates a very free moving disc. The motion wouldbe restricted in all planes of the pad depending upon these factors.Rotation is also restricted, as well as flexion or movement of the disc.The amount of compression possible is restricted or allowed according tothe pads material properties. This is true of motion towards the back orside-to-side motion. Thus, the pads 31 are always in contact and alwaysshare the load, under any adjustment of relative positioning of thehousing members 12, 14. Since motion forces the pads to be in contact,the pads 31 automatically damper loads imposed by the artificial discconstruct 10.

With specific regard to the flexibility or elasticity of the polymer orelastomer composition of the pads 31, the pads can be selected from acomposition having a durometer from 20 to 98 on the Shore OO Scale.Alternatively, the pads 31 can be selected from a composition having adurometer from 10 to 100 on the Shore A Scale. A further alternative isfor the pads 31 to be selected from a composition having a durometerfrom 22 to 75 on the Shore D Scale. In any event, the pad members 31 canbe selected during the operation and procedure by the clinician to suita specific situation. Although the pad members 31 can be pre-insertedbetween the housing members 12, 14 prior to insertion of the device 10in situ, the various configurations of the present invention can allowfor in situ replacement of the pad members 31 so as to custom select theflexibility or elasticity of the members. In this manner, the padmembers 31 are custom designed for the individual environment of theintervertebral space into which the device is being disposed.

The disc members 28 and 30, and pads 31 can be contained or locked inposition in between the housing members 12, 14 by various means. Thedisc 28, 30 can be locked to the housing members 12, 14 by a press fittaper, retaining ring, or other means. The key aspect of such lockingmechanisms is to prevent the disc members 28, 30 from moving against theupper or lower housing members 12, 14 once installed in order to preventadditional wear.

FIGS. 1 and 2 show disc members 28, 30 disposed in recesses (only thelower recess 50 is shown in FIG. 2 in an exploded view) in each of theinner surfaces 16, 18 of the housing members 12, 14. FIGS. 6 and 7 showplan views of a second embodiment of the housing member 12′, 14′,wherein each recess 50′, 52 includes a ramped surface 54, 56 leadingfrom an outer edge to the inwardly tapered recess portion 50′, 52. Theramping 54, 56 allows access of the disc members 28,30 in between thehousing members 12′, 14′ after placement of the housing members 12′, 14′in the intervertebral space. This intraoperative access of the discmembers 28, 30 allows the surgeon to test different size disc membersunder load conditions to perfectly fit the disc members in place. Suchan advantage is not obtainable with any prior art device.

An alternative mechanical mechanism for locking the disc members withinthe housing members are shown in FIG. 16. The representative housingmember 12′″ includes recess 52′. The recess 52′ includes a substantiallyarcuate peripheral undergroove 70. The groove is defined by a lipportion 72 including at least one and preferably at least two openings74, 76. The disc member 28′″ includes bayonet style flanges 78, 80extended radially outwardly therefrom, the flanges 78, 80 being shapedso as to be received through recess 74, 76. In operation the disc member28′″ can be disposed within the recess 52′ such that the flanges 78, 80align with recesses 74, 76. Once the disc member 28′″ can be rotatedthereby providing a bayonet style locking mechanism of the disc member28′″ within the housing 12′″, as shown in FIG. 17.

A further alternative embodiment of the locking mechanism is shown inFIGS. 18 and 19. The housing member 12′″ includes a substantiallyarcuate recess 52″ having an open end portion 82 extending to an edge 84of the housing member 12′″. The recess 52″ includes a lip portion 86extending about a substantial portion thereof defining an inner groove88 between the seating surface 90 of the recess 52″ and the lip portion86. Arm portions 92, 94 are extensions of the lip portion 86 but extendfrom and are separate from peripheral ends 96, 98 of the housing member12′″. The arm portions 92, 94 have a spring-like quality such that theycan be deflected outwardly from the arcuate circle defined by the recess52″. Each of the arms 92, 94 has an elbow portion 100, 102 extendingfrom each arm portion 92, 94 towards the seating surface 90,respectively. The disc member 28′″ includes a substantially arcuateperipheral, radially outwardly extending flange portion 104. The flangeportion 104 includes two abutment edges 106, 108. In operation, theflange 104 and disc member 28′″ are disposed within the annular recessor groove 88, deflecting outwardly the arms 92, 94. Once disposed in therecess 52″, as shown in FIG. 19, the elbows 100, 102 engage the abutmentsurfaces 106, 108 of the disc member 28′″ thereby locking the discmember 28′″ in place. Outward deflection of the arms 92, 94 canselectively release the disc member 28′″ from locked engagement toprovide for further adjustment of the selection of the disc memberduring an operation procedure.

Also, as best shown in FIGS. 6 and 7, the pads members 31 can bedisposed in recesses 58, 60 in the lower and upper housing members 12′,14′ respectively. It is preferable to permanently adhere the pad members31 to the housing members 12′, 14′ by use of mechanical mechanismsand/or various adhesives, such as cyanoarylates, urethanes, and othermedical grade adhesives. This list of adhesives, as with other listingsof ingredients in the present application, is merely exemplary and notmeant to be exhaustive.

Examples of mechanical mechanisms for locking the pad members 31 intorecesses in the housing members are shown in FIGS. 20-23. One suchmechanism is an undercut locking mechanism shown in FIGS. 20-22. Housingmember 12″″ includes a central recess 52 such as shown in FIG. 6 havinga ramp portion 56. The ramp portion 56 includes a centrally locatedtongue groove 57 allowing for the insertion of a spatula type deviceunder a disc member disposed within the recess 52 for releasing the discmember from the recess, similar to the use of a shoehorn type mechanism.Recesses 60′ include undercut recesses 110, 112 for locking engagementwith a peripheral flange portion 114 extending from an edge 116 of a padmember 31′. Since the pad member is made from a deflectable material,the flange portion 114 can be force-fit into and seated within theundercut 110, 112. The undercut locking mechanism effectively preventsthe pad member 31′ from disengagement with the housing member 12″″ insitu. Of course, the upper flange 118 would be locked within a similarundercut locking detail of recesses within the opposing housing member(not shown).

An alternative locking mechanism between the pad member and housingmember can be a tongue-and-groove relationship as shown in FIG. 23.Either the pad or the housing can include the tongue portion 122 and theother pad and housing members can include the groove 124. In otherwords, either of the locking members can include the tongue 122 and theother of the members being locked would include the groove 124. Analternative of this or the other locking mechanism shown is that therecess and/or pad can include multiple grooves or slots as well asmultiple tongues.

The various recesses or pockets 50′, 52, 58, 60 can be of differentrelative sizes and shapes. For example, the upper housing member 12′ mayhave a larger recess or pocket for seating a relatively larger one ofsaid discs 28 and the lower housing member 14′ may be include a smaller(larger and smaller referring to diameter of the annular recess) of therecesses or pockets for seating a relatively smaller one of the lowerdisc 30, thereby providing for an increased range of motion at thebearing surface interface.

The various Figures show that the outer surfaces 20, 22 of the variousembodiments of the housing members 12, 14 can include flanges generallyindicated at 60. The flanges 60 or fins, as they are sometimes referredto in the art, provide a mechanism for fixation to the intervertebralsurfaces. Various embodiments, such as those shown in FIGS. 1 and 2 aredual fin constructs. Other embodiments, such as those shown in FIGS. 8,12, and 13 are single fin or single flange constructs. Depending uponthe nature of the surfaces to which the outer surfaces 20, 22 are toabut, the surgeon can select various flange or fin configurations.Additionally, the fins 60 can be located in alternative positions,either centrally as shown in many of the Figures, or peripherally, asshown in FIG. 14, for a specific use with anterior extension plates, aswith screw fixations. The flanges, such as flange 60′″″″ can include abore 62 therethrough, which can be either a smooth surface or threadeddepending on its intended use.

The outer surfaces 20, 22 can be smooth, which allows for easierrevision as it allows for minimal to no ingrowth or they can betextured. Texturing of the outer surfaces 20, 22 allows ingrowth forlong-term fixation of the assembly 10. Porous coatings, plasma spray,grit blasting, machining, chemical etching, or milling are examples oftechniques for creating ingrowth capable surfaces. Coatings that enhancebone growth can also be applied. Examples of such coatings arehyroxyapatite and bone morphogenic proteins.

FIGS. 20 and 21 provide structure for further rotational stability ofthe device in situ. The housing member 12″″ includes pointed portions126, 128 extending from the outer surface 20′ thereof. The point members126, 128 function in conjunction with the flange portion 61′ to engagean opposing vertebral surface. The point portions 126, 128 beingdisposed radially peripherally from the centrally disposed flange 61′provide at least a three-point engagement of the vertebral surfacethereby preventing rotation of the housing member 12″″ relative thereto.Of course, the point portions 126, 128 can be in made in variousconfigurations and extend various amounts from the outer surface 20′ tobe custom suited to a specific vertebrae surface shape.

Various methods can be utilized for insertion of the present inventionin situ. For example, an assembled device 10 as shown in FIG. 1, can bedisposed between the intervertebral spaces during surgery, aftercalculation of space, depth, and height. Alternatively, opposing housingmembers 12, 14 can be disposed between the intervertebral spaces andpads 31 and disc members 24, 26 can be tested in situ prior to fixationthereof to allow for custom sizing. Accordingly, the present inventionbroadly provides a method of assembling an artificial intervertebraldisc 10 in vivo by inserting upper and lower housing members 12, 14 intoan intervertebral space and disposing cushioning pads 31 between theinner surfaces 16, 18 of the housing members 12, 14, thereby placing thepads in compression. The pair of disc members 28, 30 are insertedbetween the inner surfaces of the plates 16, 18. The disc members 28, 30have abutting low friction surfaces 24, 26 therebetween. The discmembers 28, 30 are surrounded by the pads 31, whereby the disc members28, 30 and pads 31 are under compressive forces and share suchcompressive forces. This step of the bearing surfaces 24, 26 and shockabsorbing pads 31 sharing absorption of the compressive forces andlimiting the relative movement of the housing members 12, 14 is anadvantage not found in the prior art.

1-51. (canceled)
 52. A method of assembling an artificial intervertebraldisc in vivo by inserting upper and lower housing members into anintervertebral space; disposing cushioning pads between inner surfacesof the housing members, placing the pads in compression; and inserting apair of disc members between the inner surfaces of the plates, the discmembers having abutting low friction surfaces therebetween the discmembers being effectively surrounded by the pads whereby the discmembers and pads share compression forces.
 53. A method according toclaim 52 further including the steps of aligning the housing membersrelative to each other by adjusting the relative height of thecushioning pads to maintain spinal curvatures.
 54. A method according toclaim 52 further including the steps of creating a no load condition onthe cushioning pads by adjusting the relative heights of the cushioningpads.
 55. A method according to claim 52 further including the steps ofadjusting the relative range of motion of the housing members byadjusting for relative elasticity of the cushioning pad members.
 56. Amethod of separating opposing vertebrae at an intervertebral space by:engaging artificial bearing surfaces between the intervertebral surfaceswhile allowing low friction and compression resistant movement of thebearing surfaces relative to each other; sharing absorption of thecompressive forces with at least one load bearing pad disposed about atleast a portion of the bearing surfaces; and limiting the relativemovement of the bearing surfaces.
 57. A method of artificially spacingapart intervertebral surfaces by: disposing housing members separated bybearing surfaces surrounded by pad members between the intervertebralspaces; and sharing compressive loads between the bearing surfaces andpad members.
 58. The method as set forth in claim 57 further includingthe steps of limiting compression of the pad members by the bearingsurfaces being noncompressible and limiting motion of the bearingsurfaces by the pad members having limited elasticity.
 59. The method asset forth in claim 58 further including steps of adjusting theelasticity of the pad members intraoperatively to adjust the extentmovement of the bearing surfaces.
 60. An artificial intervertebral disccomprising; noncompressible bearing surface means for allowing motionbetween two bearing surfaces while limiting compression thereof; andcompression absorbing means for sharing compressive forces with saidbearing surface means while limiting motion of said bearing surfacemeans.
 61. A disc member for an artificial intervertebral disc, saidartificial intervertebral disc comprising housing members includingspaced inner surfaces facing each other and oppositely facing outersurfaces for engaging spaced apart intervertebral surfaces, said discmember comprising bearing means extending from each of said innersurfaces for engaging each other while allowing for low friction andcompression resistance relative to each other while under compression.62. A pad member for an artificial intervertebral disc, said artificialintervertebral disc comprising comprising housing members includingspaced inner surfaces facing each other and oppositely facing outersurfaces for engaging spaced apart intervertebral surfaces and bearingmeans extending from each of said inner surfaces for engaging each otherwhile allowing for low friction and compression resistance relative toeach other while under compression, said pad member comprising loadsharing means disposed between said inner surfaces and about at least aportion of said bearing means for sharing absorption of compressiveloads with said bearing means while limiting the relative movement ofsaid housing members.
 63. A method of assembling an artificialintervertebral disc in situ by: inserting a pair of housing membersconnected together by a pair of load sharing pads into an intervertebralspace and then inserting a pair of bearing surfaces between the housingmembers and between the load sharing pads whereby the disc members andpads share compression forces.
 64. A method of artificially spacingapart opposing intervertebral spaces by sharing compressive forcesbetween bearing surfaces and load sharing pad members.
 65. A loadsharing pad member comprising a body portion and at least one peripheralflange portion for engaging recess in a portion of a vertebral dischousing member.
 66. A load sharing pad member comprising a body portionand at least one peripheral recess portion for being engaged by a flangeportion in a product of a vertebral disc housing member.
 67. A vertebraldisc housing member including one centrally located recess portion andat least one other recesses portion disposed peripherally relative tosaid centrally located recess portion.